Ion exchangeable glass article for three-dimensional forming

ABSTRACT

An alkali aluminosilicate glass that can be chemically strengthened and formed into a three dimensional shape. The glass has a softening point of less than about 825° C. and a high temperature coefficient of thermal expansion of less than about 30 parts per million (ppm)/° C. The glass may be ion exchanged after the three dimensional shape is formed. When ion exchanged, the glass has a surface layer that is under a compressive stress of at least about 700 MPa.

This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/945,430, filed on Feb. 27, 2014, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates to a glass that is formable into three dimensional shapes. More particularly, the disclosure relates to an ion exchanged glass that is formable into three dimensional shapes. Even more particularly, the disclosure relates to such glasses having a low softening point.

Shaped glass articles are entering use as outer covers or enclosure elements in consumer electronics such as mobile phones and tablets. Currently, these articles are made by molding the glass.

SUMMARY

The present disclosure meets these and other needs by providing an alkali aluminosilicate glass that can be chemically strengthened and formed into a three dimensional shape. The glass has a softening point of less than about 825° C. and a high temperature coefficient of thermal expansion of less than about 30 parts per million (ppm)/° C. The glass may be ion exchanged after the three dimensional shape is formed. When ion exchanged, the glass has a surface layer that is under a compressive stress of at least about 700 MPa.

Accordingly, one aspect of the disclosure is to provide a glass article comprising at least about 50 mol % SiO₂, at least about 8 mol % Al₂O₃, at least about 1 mol % P₂O₅, and at least about 12 mol % Na₂O. The glass article is ion exchangeable and has a softening point of less than or equal to about 825° C., and a high temperature coefficient of thermal expansion of less than or equal to 30 ppm/° C.

A second aspect of the disclosure is to provide a glass article comprising at least about 50 mol % SiO₂, at least about 8 mol % Al₂O₃, at least about 1 mol % P₂O₅, and at least about 12 mol % Na₂O. The glass article is ion exchanged and has a compressive layer extending from a surface of the glass article to a depth of layer within the article, wherein the compressive layer has a maximum compressive stress of at least about 700 MPa. The glass article has a softening point of less than or equal to about 825° C. and a high temperature coefficient of thermal expansion of less than or equal to 29 ppm/° C.

These and other aspects, advantages, and salient features will become apparent from the following detailed description, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of instantaneous coefficient of thermal expansion as a function of temperature for sample 20 in Table 1;

FIG. 2 is a cross-sectional schematic view of dish-shaped glass articles; and

FIG. 3 is a cross-sectional schematic view of a planar ion exchanged glass article.

DETAILED DESCRIPTION

In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that, unless otherwise specified, terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as comprising at least one of a group of elements and combinations thereof, it is understood that the group may comprise, consist essentially of, or consist of any number of those elements recited, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a group of elements or combinations thereof, it is understood that the group may consist of any number of those elements recited, either individually or in combination with each other. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range as well as any ranges therebetween. As used herein, the indefinite articles “a,” “an,” and the corresponding definite article “the” mean “at least one” or “one or more,” unless otherwise specified. It also is understood that the various features disclosed in the specification and the drawings can be used in any and all combinations.

As used herein, the terms “glass article” and “glass articles” are used in their broadest sense to include any object made wholly or partly of glass. Unless otherwise specified, all compositions are expressed in terms of mole percent (mol %). High temperature coefficients of thermal expansion (high temperature CTE) are expressed in terms of part per million (ppm) per degree Celsius (ppm/° C.), and represent a value measured in the high temperature plateau region of the instantaneous CTE vs. temperature curve.

Unless otherwise specified, all temperatures are expressed in terms of degrees Celsius (° C.). As used herein the term “softening point” refers to the temperature at which the viscosity of a glass is approximately 10⁷⁶ poise (P), the term “anneal point” refers to the temperature at which the viscosity of a glass is approximately 10^(13.2) poise, the term “200 poise temperature (T^(200P))” refers to the temperature at which the viscosity of a glass is approximately 200 poise, the term “1 poise temperature (T^(200P))” refers to the temperature at which the viscosity of a glass is approximately 200 poise, the term “10¹¹ poise temperature” refers to the temperature at which the viscosity of a glass is approximately 10¹¹ poise, the term “35 kP temperature (T^(35kP))” refers to the temperature at which the viscosity of a glass is approximately 35 kilopoise (kP), and the term “160 kP temperature (T^(160kP))” refers to the temperature at which the viscosity of a glass is approximately 160 kP.

As used herein, the term “zircon breakdown temperature” or “T^(breakdown)” refers to the temperature at which zircon—which is commonly used as a refractory material in glass processing and manufacture—breaks down to form zirconia and silica, and the term “zircon breakdown viscosity” refers to the viscosity of the glass at T^(breakdown). The term “liquidus viscosity” refers to the viscosity of a molten glass at the liquidus temperature, wherein the liquidus temperature refers to the temperature at which crystals first appear as a molten glass cools down from the melting temperature, or the temperature at which the very last crystals melt away as temperature is increased from room temperature.

It is noted that the terms “substantially” and “about” may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. Thus, a glass that is “substantially free of lithium,” for example is one in which lithium and compounds formed by lithium is not actively added or batched into the glass, but may be present in very small amounts as a contaminant.

Vickers crack initiation thresholds described herein are determined by applying and then removing an indentation load to the glass surface at a rate of 0.2 mm/min. The maximum indentation load is held for 10 seconds. The indentation cracking threshold is defined at the indentation load at which 50% of 10 indents exhibit any number of radial/median cracks emanating from the corners of the indent impression. The maximum load is increased until the threshold is met for a given glass composition. All indentation measurements are performed at room temperature in 50% relative humidity.

As used herein, “maximum compressive stress” refers to the highest compressive stress value measured within the compressive layer. In some embodiments, the maximum compressive stress is located at the surface of the glass and may appear as a “spike” in the compressive stress profile. In other embodiments, the maximum compressive stress may occur at a depth below the surface, giving the compressive profile the appearance of a “buried peak.” Compressive stress and depth of layer are measured using those means known in the art. Such means include, but are not limited to, measurement of surface stress (FSM) using commercially available instruments such as the FSM-6000, manufactured by Luceo Co., Ltd. (Tokyo, Japan), or the like, and methods of measuring compressive stress and depth of layer are described in ASTM 1422C-99, entitled “Standard Specification for Chemically Strengthened Flat Glass,” and ASTM 1279.19779 “Standard Test Method for Non-Destructive Photoelastic Measurement of Edge and Surface Stresses in Annealed, Heat-Strengthened, and Fully-Tempered Flat Glass,” the contents of which are incorporated herein by reference in their entirety. Surface stress measurements rely upon the accurate measurement of the stress optical coefficient (SOC), which is related to the birefringence of the glass and expressed in nm/mm/MPa. SOC in turn is measured by those methods that are known in the art, such as fiber and four point bend methods, both of which are described in ASTM standard C770-98 (2008), entitled “Standard Test Method for Measurement of Glass Stress-Optical Coefficient,” the contents of which are incorporated herein by reference in their entirety, and a bulk cylinder method.

As used herein, the term “three dimensional shape” refers to a shape or form other than a flat sheet. The three dimensional shape does not lie in a plane. Non-limiting examples of three dimensional glass articles are shown in FIG. 2. Dish-shaped article 200 has two major surfaces 202, 204 each of which has a substantially flat or planar portion 210, bounded on either end (or, alternatively, on both ends) by a curved portion 220 to provide a dish-shaped profile or appearance. In other embodiments, dish-shaped article 230 has only one major surface 234 having a substantially flat or planar portion 210, bounded on either end (or, alternatively, on both ends) by a curved portion 220. The remaining major surface 232 is substantially flat or planar.

Referring to the drawings in general and to FIG. 1 in particular, it will be understood that the illustrations are for the purpose of describing particular embodiments and are not intended to limit the disclosure or appended claims thereto. The drawings are not necessarily to scale, and certain features and certain views of the drawings may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

In order for a glass to be formable into a three dimensional shape for use a cover glass in hand held electronic devices, the glass should have a low softening point to facilitate shaping and a sufficiently low coefficient of thermal expansion (CTE) at high temperature to prevent cracking. In addition, the glass should be ion exchangeable to achieve a surface compressive stress that is sufficient to prevent damage due to impact.

Described herein is a family of glasses that are fusion formable, ion exchangeable, and formable into three dimensional shapes. The glass comprises at least about 50 mol % SiO₂; at least about 8 mol % Al₂O₃; at least about 1 mol % P₂O₅; and at least about 12 mol % Na₂O, and has a softening point of less than or equal to about 825° C. and a high temperature coefficient of thermal expansion (high temperature CTE) of less than or equal to about 30 ppm/° C. In some embodiments, the softening point of the glasses described herein is less than or equal to about 800° C. and, in still other embodiments, less than or equal to about 775° C.

As previously mentioned, the high temperature coefficient of thermal expansion is taken as the instantaneous coefficient of thermal expansion of the glass at high temperatures. FIG. 1 is a plot of instantaneous coefficient of thermal expansion as a function of temperature for sample 20 in Table 1. The high temperature CTE of this glass sample is the instantaneous CTE at the high temperature plateau occurring at about 675° C. In some embodiments, the high temperature coefficient of thermal expansion is less than or equal to about 29 ppm/° C. and, in other embodiments, less than or equal to about 27 ppm/° C. Exemplary compositions of these glasses are listed in Table 1. Softening points, high temperature coefficients of thermal expansion, and other physical properties of the glasses listed in Table 1, including strain points, anneal points, T^(200P), 10¹¹ poise temperature, T^(35kP), T^(breakdown), zircon breakdown viscosity, T^(160kP) liquidus temperature, liquidus viscosity, refractive index, and SOC, are listed in Table 2.

TABLE 1 Compositions of glasses. Analyzed Composition (mol %) 1 2 3 4 5 6 SiO₂ 58.30 58.48 58.41 58.71 59.06 59.01 Al₂O₃ 15.60 14.45 13.56 12.64 11.74 10.69 P₂O₅ 4.76 4.76 4.82 4.74 4.67 4.68 B₂O₃ 5.05 5.14 5.16 4.92 4.79 4.81 Na₂O 13.31 14.27 15.17 16.08 16.73 17.85 ZnO 2.98 2.90 2.88 2.92 3.00 2.96 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 (R₂O + RO + B₂O₃)/ 0.27 0.29 0.30 0.31 0.33 0.34 (Al₂O₃ + P₂O5 + SiO₂) Analyzed Composition (mol %) 7 8 9 10 11 12 SiO₂ 57.73 57.88 57.91 57.82 58.19 58.27 Al₂O₃ 15.51 14.52 13.63 12.61 11.71 10.71 P₂O₅ 4.87 4.81 4.77 4.88 4.75 4.88 B₂O₃ 4.97 4.92 4.89 4.82 4.53 4.49 Na₂O 13.09 14.06 14.90 15.95 16.92 17.66 ZnO 3.82 3.80 3.90 3.93 3.90 3.99 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 (R₂O + RO + B₂O₃)/ 0.28 0.30 0.31 0.33 0.34 0.35 (Al₂O₃ + P₂O5 + SiO₂) Analyzed Composition (mol %) 13 14 15 16 17 18 SiO₂ 56.52 55.11 56.69 56.69 57.11 57.36 Al₂O₃ 15.49 14.52 13.53 12.55 11.62 10.70 P₂O₅ 4.83 4.88 4.81 4.78 4.72 4.70 B₂O₃ 5.00 5.08 4.87 4.90 4.63 4.47 Na₂O 13.08 14.35 14.93 15.91 16.80 17.62 ZnO 5.09 6.05 5.16 5.17 5.12 5.15 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 (R₂O + RO + B₂O₃)/ 0.30 0.34 0.33 0.35 0.36 0.37 (Al₂O₃ + P₂O5 + SiO₂) Analyzed Composition (mol %) 19 20 21 22 23 24 SiO₂ 58.31 56.68 58.64 57.51 57.69 53.73 Al₂O₃ 12.68 12.53 12.50 12.49 12.76 14.52 P₂O₅ 4.70 4.87 4.87 4.88 4.72 4.93 B₂O₃ 4.69 4.93 5.03 5.12 4.47 5.04 Na₂O 16.72 18.12 17.02 18.03 18.48 17.89 ZnO 2.90 2.87 1.95 1.97 1.88 3.88 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 (R₂O + RO + B₂O₃)/ 0.32 0.35 0.32 0.34 0.33 0.37 (Al₂O₃ + P₂O5 + SiO₂) Analyzed Composition (mol %) 25 26 27 28 29 30 SiO₂ 53.61 53.61 54.12 54.08 54.93 56.37 Al₂O₃ 14.50 14.42 14.71 14.58 14.73 12.59 P₂O₅ 3.95 5.87 2.95 6.66 4.75 5.04 B₂O₃ 6.01 4.06 6.64 3.05 4.53 5.01 Na₂O 17.93 17.96 17.61 17.81 17.64 19.09 ZnO 4.00 4.08 3.97 3.83 3.43 1.90 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 (R₂O + RO + B₂O₃)/ 0.39 0.35 0.39 0.33 0.34 0.35 (Al₂O₃ + P₂O5 + SiO₂) Analyzed Composition (mol %) 31 32 33 34 35 36 SiO₂ 56.21 56.89 56.06 56.80 57.04 53.13 Al₂O₃ 12.58 12.73 12.53 12.35 12.85 15.15 P₂O₅ 5.98 4.01 7.14 2.88 2.08 2.92 B₂O₃ 4.02 5.49 2.94 6.73 7.24 6.51 Na₂O 19.31 18.93 19.41 19.29 18.72 18.84 ZnO 1.90 1.95 1.92 1.95 2.07 3.46 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 (R₂O + RO + B₂O₃)/ 0.34 0.36 0.32 0.39 0.39 0.40 (Al₂O₃ + P₂O5 + SiO₂) Analyzed Composition (mol %) 37 38 39 40 41 42 SiO₂ 53.02 52.76 52.98 52.69 53.86 54.33 Al₂O₃ 15.13 15.21 15.29 15.76 15.22 14.99 P₂O₅ 2.92 2.94 2.94 3.00 2.06 2.08 B₂O₃ 6.61 6.32 6.29 6.39 7.49 7.41 Na₂O 18.37 18.83 19.10 18.79 17.66 16.95 ZnO 3.94 3.94 3.40 3.37 3.71 4.23 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 (R₂O + RO + B₂O₃)/ 0.41 0.41 0.40 0.40 0.41 0.40 (Al₂O₃ + P₂O5 + SiO₂) Analyzed Composition (mol %) 43 44 45 46 47 48 49 SiO₂ 54.15 53.96 53.93 54.07 53.98 53.93 53.91 Al₂O₃ 15.47 14.59 15.19 14.52 14.74 14.69 14.80 P₂O₅ 1.58 2.43 2.44 1.45 2.49 2.47 2.45 B₂O₃ 7.78 7.24 7.17 8.18 7.21 7.27 7.08 Na₂O 17.27 17.88 17.81 17.83 17.56 17.67 17.73 ZnO 3.75 3.89 3.46 3.95 4.02 3.97 3.89 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.14 (R₂O + RO + B₂O₃)/ 0.40 0.41 0.40 0.43 0.40 0.41 0.40 (Al₂O₃ + P₂O5 + SiO₂)

TABLE 2 Physical properties of the glasses listed in Table 1. Sample 1 2 3 4 5 6 Softening Point 882.1 841.5 821.9 805.2 805.7 808.6 CTE 19.38 22.34 24 27 27 27.22 Strain Point 559 528 521 520 524 524 Anneal Point 614 579 568 562 562 563 T^(200P) 1620 1601 1584 1557 1532 1487 10¹¹ P 705 666 650 638 633 636 Temperature T^(35k-P) 1184 1156 1131 1097 1070 1033 T^(breakdown) >1290 Zircon <2528 Breakdown Viscosity (P) T^(160kP) 1099 1069 1044 1011 985 951 Liquidus 870 Temperature Liquidus 3.74 × 10⁶ Viscosity (P) Refractive 1.4949 1.4956 1.4957 1.4969 1.4966 1.4957 Index SOC 3.472 3.436 3.434 3.341 3.319 3.282 Sample 7 8 9 10 11 12 Softening Point 854.2 847.6 815.8 810.8 817.5 815.7 CTE 20.6 22.1 22.2 25.8 27.9 25.5 Strain Point 541 532 522 532 528 Anneal Point 595 583 565 571 569 T^(200P) 1593 1582 1570 1547 1515 1485 10¹¹ P 685 671 642 644 644 Temperature T^(35kP) 1165 1146 1128 1094 1066 1037 T^(breakdown) Zircon Breakdown Viscosity (P) T^(160kP) 1081 1060 1041 1009 982 955 Liquidus Temperature Liquidus Viscosity (P) Refractive Index 1.497 1.4971 1.4973 1.499 1.4985 1.496 SOC 3.482 3.505 3.504 3.442 3.354 3.337 Sample 13 14 15 16 17 18 Softening Point 843.8 832.6 813.8 834.6 819.9 821 CTE 20.7 21.44 21.9 24.25 28.14 26.33 Strain Point 545 530 520 535 536 535 Anneal Point 595 579 566 575 577 576 T^(200P) 1558 1565 1541 1517 1495 1461 10¹¹ P 679 663 646 650 651 651 Temperature T^(35kP) 1142 1131 1109 1078 1057 1025 T^(breakdown) Zircon Breakdown Viscosity (P) T^(160kP) 1059 1047 1024 995 976 945 Liquidus Temperature Liquidus Viscosity (P) Refractive Index SOC 3.494 3.486 3.497 3.467 3.408 3.349 Sample 19 20 21 22 23 24 Softening Point 798.1 779.2 780.7 771.3 766.4 786.4 CTE 29 29 29 28 27 27.2 Strain Point 524 514 513 513 512 515 Anneal Point 566 554 557 555 553 558 T^(200P) 1543 1500 1546 1514 1504 1492 10¹¹ P 640 625 632 627 624 633 Temperature T^(35kP) 1094 1055 1085 1057 1052 1067 T^(breakdown) Zircon Breakdown Viscosity (P) T^(160kP) 1007 971 997 972 968 986 Liquidus Temperature Liquidus Viscosity (P) Refractive Index 1.4958 1.4965 1.4968 1.5018 SOC 3.321 3.327 3.252 3.246 3.234 3.349 Sample 25 26 27 28 29 30 Softening 777.9 792.6 770.4 799.5 803.1 759.9 Point CTE 27.3 26 28.56 26 25.4 29.15 Strain Point 514 519 516 518 527 509 Anneal 557 562 559 563 572 549 Point T^(200P) 1483 1513 1480 1522 1517 1488 10¹¹ P 631 637 631 641 649 618 Temperature T^(35kP) 1056 1086 1054 1093 1089 1038 T^(breakdown) 1110 Zircon 14008 Breakdown Viscosity (P) T^(160kP) 976 1003 974 1011 1006 955 Liquidus 820 Temperature Liquidus 7.09 × 10⁶ Viscosity (P) Refractive 1.5037 1.5003 1.5056 1.4989 1.5001 1.49642 Index SOC 3.407 3.321 3.369 3.249 3.393 3.206 Sample 31 32 33 34 35 36 Softening 771.5 751.4 773.4 737 738.6 770.7 Point CTE 27.51 30.9 28.6 34.29 35.2 29.61 Strain Point 510 515 506 516 518 522 Anneal 551 556 548 555 558 564 Point T^(200P) 1496 1480 1512 1447 1465 1465 10¹¹ P 623 624 621 619 622 634 Temperature T^(35kP) 1048 1029 1066 999 1008 1048 T^(breakdown) Zircon Breakdown Viscosity (P) T^(160kP) 965 946 982 919 928 969 Liquidus Temperature Liquidus Viscosity (P) Refractive 1.49431 1.49889 1.49445 1.50233 1.50475 1.5044 Index SOC 3.124 3.181 3.077 3.172 3.217 3.388 Sample 37 38 39 40 41 42 Softening 770.2 765.6 773.3 785.3 772.3 769 Point CTE 29.62 28.92 29.75 28.26 28.27 29.42 Strain Point 521 522 524 523 517 509 Anneal 563 563 567 566 561 551 Point T^(200P) 1465 1450 1468 1477 1468 1456 10¹¹ P 634 632 638 639 634 623 Temperature T^(35kP) 1047 1038 1046 1057 1050 1039 T^(breakdown) Zircon Breakdown Viscosity (P) T^(160kP) 968 960 966 977 968 958 Liquidus 820 795 Temperature Liquidus 5.29 × 10⁶ 7.88 × 10⁶ Viscosity (P) Refractive 1.5052 1.5057 1.5089 1.5046 1.5087 1.5079 Index SOC 3.424 3.425 3.344 3.369 3.41 3.434 Sample 43 44 45 46 47 48 49 Softening 778 762.4 772.3 759 766.8 763.5 767.7 Point CTE 28.41 28.8 28.3 30.5 28.6 30.04 28.08 Strain Point 521 517 518 516 513 511 516 Anneal 565 559 562 558 555 553 558 Point T^(200P) 1476 1466 1475 1451 1467 1479 1443 10¹¹ P 638 629 635 628 626 624 629 Temperature T^(35kP) 1062 1043 1061 1032 1048 1040 1039 T^(breakdown) 1075 1075 Zircon 22121 18989 Breakdown Viscosity (P) T^(160kP) 980 963 979 952 968 962 960 Liquidus 805 1090 770 Temperature Liquidus 1.12 × 10⁷ 1.73 × 10⁴ 2.15 × 10⁷ Viscosity (P) Refractive 1.5065 1.5064 1.5052 1.5086 1.5065 1.5069 1.5062 Index SOC 3.419 3.439 3.397 3.419 3.41 3.41 3.408

In some embodiments, the glasses described herein may be formed into a three dimensional shape using those means known in the art, including molding or the like. Non-limiting examples of such three dimensional shapes include those articles in which at least one surface has a dish-shaped, curved, convex, or concave profile. Dish-shaped articles may have a substantially flat portion bounded on at least one side by a curved portion. Non-limiting examples of dish-shaped glass ceramic articles are schematically shown in cross-sectional views in FIG. 2. Dish-shaped article 200 has two major surfaces 202, 204 each of which has a substantially flat or planar portion 210, bounded on either end (or, alternatively, on both ends) by a curved portion 220 to provide a dish-shaped profile or appearance. In other embodiments, dish-shaped article 230 has only one major surface 234 having a substantially flat or planar portion 210, bounded on either end (or, alternatively, on both ends) by a curved portion 220. The remaining major surface 232 is substantially flat or planar.

Ion exchange is widely used to chemically strengthen glasses. In one particular example, alkali cations within a source of such cations (e.g., a molten salt, or “ion exchange,” bath) are exchanged with smaller alkali cations within the glass to achieve a layer that is under a compressive stress (CS) near the surface of the glass. The compressive layer extends from the surface to a depth of layer (DOL) within the glass. In the glasses described herein, for example, potassium ions from the cation source are exchanged for sodium ions within the glass during ion exchange by immersing the glass in a molten salt bath comprising a potassium salt such as, but not limited to, potassium nitrate (KNO₃). Other potassium salts that may be used in the ion exchange process include, but are not limited to, potassium chloride (KCl), potassium sulfate (K₂SO₄), combinations thereof, and the like.

A cross-sectional schematic view of a planar ion exchanged glass article is shown in FIG. 3. Glass article 300 has a thickness t, first surface 310, and second surface 312. While the embodiment shown in FIG. 3 depicts glass article 300 as a flat planar sheet or plate, glass article may have other configurations, such as three dimensional shapes or non-planar configurations. Glass article 300 has a first compressive layer 320 extending from first surface 310 to a depth of layer d₁ into the bulk of the glass article 300. In the embodiment shown in FIG. 3, glass article 300 also has a second compressive layer 322 extending from second surface 312 to a second depth of layer d₂. Glass article also has a central region 330 that extends from d₁ to d₂. Central region 330 is under a tensile stress or central tension (CT), which balances or counteracts the compressive stresses of layers 320 and 322. The depth d₁, d₂ of first and second compressive layers 320, 322 protects the glass article 300 from the propagation of flaws introduced by sharp impact to first and second surfaces 310, 312 of glass article 300, while the compressive stress minimizes the likelihood of a flaw penetrating through the depth d₁, d₂ of first and second compressive layers 320, 322.

The glass articles described herein may be ion exchanged after being formed into a three dimensional shape. In non-limiting examples, the glass is annealed at the temperature defined by the 10^(13.2) poise viscosity of the glass and ion exchanged in a molten potassium nitrate bath for either four, eight, or 10 hours. The ion exchange bath may comprise nearly 100% KNO₃ by weight. In some embodiments, the ion exchange bath may comprise eat least about 95% KNO₃ by weight and, in other embodiments, at least about 92% KNO₃ by weight. The compressive layer comprises K₂O and has a maximum compressive stress of at least about 700 MPa. In some embodiments, the compressive layer or layers (320, 322 in FIG. 3) may have a maximum compressive stress CS of at least about 700 MPa. In other embodiments the maximum compressive stress is at least about 800 MPa and, in still other embodiments, at least about 900 MPa. The depth of layer DOL (d₁, d₂ in FIG. 3) of each of compressive layers 320, 322 is, in some embodiments, at least 20 μm. In other embodiments, the depth of layer is at least about 30 μm.

Table 3 lists compressive stresses (CS), depths of layer (DOL), and Vickers crack indentation thresholds obtained by ion exchange of the glasses listed in Table 1. In a first set of ion exchange experiments, 1 mm thick samples were first annealed at the temperature defined by the 10^(13.2) poise viscosity of the glass and then ion exchanged at 410° C. in a KNO₃ bath for four hours. In a second set of experiments, 1 mm thick samples were first heated to the 10¹¹ poise temperature and quenched to room temperature in order to simulate the thermal history produced in the fusion draw process. These quenched samples were then ion exchanged at 410° C. in a KNO₃ bath for four hours.

TABLE 3 Compressive stresses (CS), depths of layer (DOL), and Vickers crack indentation thresholds obtained by ion exchange of the glasses listed in Table 1. Sample 1 2 3 4 5 6 Annealed CS (MPa) 776 803 873 903 803 720 DOL (μm) 31 34 34 36 38 39 Indentation 30-40 30-40 20-30 <10 <10 <10 threshold (kgf) Heated at 10¹¹ poise, quenched CS (MPa) 707 690 773 762 676 568 DOL (μm) 36 38 42 44 46 48 Indentation 30-40 30-40 20-30 <10 <10 <10 threshold (kgf) Sample 7 8 9 10 11 12 Annealed CS (MPa) 776 803 873 903 803 787 DOL (μm) 31 34 34 36 38 36 Indentation 30-40 30-40 20-30 <10 <10 <10 threshold (kgf) Heated at 10¹¹ poise, quenched CS (MPa) 707 690 773 762 676 606 DOL (μm) 36 38 42 44 46 45 Indentation 30-40 30-40 20-30 <10 <10 10-20 threshold (kgf) Sample 13 14 15 16 17 18 Annealed CS (MPa) 1045 DOL (μm) 32 Indentation <10 threshold (kgf) Heated at 10¹¹ poise, quenched CS (MPa) 689 705 913 825 808 649 DOL (μm) 29 33 41 38 41 43 Indentation 20-30 20-30 10-20 20-30 <10 <10 threshold (kgf) Sample 19 20 21 22 23 24 Annealed Sample 19 20 21 22 23 24 CS (MPa) 866 809 814 736 739 1010 DOL (μm) 38 40 41 44 44 34 Indentation <10 <10 10-20 <10 <10 <10 threshold (kgf) Heated at 10¹¹ poise, quenched CS (MPa) 744 693 666 602 620 843 DOL (μm) 46 47 49 50 54 43 Indentation <10 <10 10-20 <10 <10 <10 threshold (kgf) Sample 25 26 27 28 29 Annealed CS (MPa) 1034 994 885 991 DOL (μm) 30 25 46 36 Indentation <10 <10 10-20 <10 10-20 threshold (kgf) Heated at 10¹¹ poise, quenched CS (MPa) 851 828 830 776 813 DOL (μm) 37 49 33 54 46 Indentation <10 <10 10-20 <10 10-20 threshold (kgf)

In some embodiments, the glasses described herein comprise: at least about 50 mol % SiO₂ (i.e., SiO₂≧50 mol %); from about 10 mol % to about 20 mol % Al₂O₃ (i.e., 10 mol %≦Al₂O₃≦20 mol %); from about 1 mol % to about 8 mol % P₂O₃ (i.e., 1 mol %≦P₂O≦8 mol %); from about 2 mol % to about 10 mol % B₂O₃ (i.e., 2 mol %≦B₂O₃≦10 mol %); and from about 14 mol % to about 20 mol % Na₂O (i.e., 14 mol %≦Na₂O≦20 mol %). The glass may further comprise from about 1 mol % to about 7 mol % ZnO (i.e., 1 mol %≦ZnO≦7 mol %).

In some embodiments, Al₂O₃(mol %)>P₂O₅(mol %)+B₂O₃(mol %) and Al₂O₃(mol %)>B₂O₃(mol %) and, in some embodiments, Na₂O(mol %)>Al₂O₃(mol %). In some embodiments, 0.3≦[(R₂O(mol %)+RO(mol %)+B₂O₃(mol %)]/[Al₂O₃(mol %)+P₂O₅ (mol %)+SiO₂ (mol %)]≦0.45, where R₂O are monovalent cation oxides and RO are divalent cation oxides. In some embodiments, the glass is substantially free of at least one of lithium, potassium, alkaline earth metals, and compounds thereof.

Each of the oxide components of the base and ion exchanged glasses described herein serves a function. Silica (SiO₂), for example, is the primary glass forming oxide, and forms the network backbone for the molten glass. Pure SiO₂ has a low CTE and is alkali metal-free. Due to its extremely high melting temperature, however, pure SiO₂ is incompatible with the fusion draw process. The viscosity curve is also much too high to match with any core glass in a laminate structure. In some embodiments, the glasses described herein comprise at least about 50 mol % SiO₂, in other embodiments, from about 50 mol % to about 65 mol % SiO₂, and, in other embodiments, from about 50 mol % to about 60 mol % SiO₂.

In addition to silica, the glasses described herein comprise the network formers Al₂O₃ and B₂O₃ to achieve stable glass formation, low CTE, low Young's modulus, low shear modulus, and to facilitate melting and forming. Like SiO₂, Al₂O₃ contributes to the rigidity to the glass network. Alumina can exist in the glass in either fourfold or fivefold coordination. In some embodiments, the glasses described herein comprise from about 10 mol % to about 20 mol % Al₂O₃ and, in particular embodiments, from about 12 mol % to about 16 mol % mol % Al₂O₃.

Phosphorous pentoxide (P₂O₅) is a network former incorporated in these glasses. P₂O₅ adopts a quasi-tetrahedral structure in the glass network; i.e., it is coordinated with four oxygen atoms, but only three of which are connected to the rest of the network. The fourth oxygen is a terminal oxygen that is doubly bound to the phosphorous cation. Association of boron with phosphorus in the glass network can lead to a mutual stabilization of these network formers in tetrahedral configurations, as with SiO₂. Like B₂O₃, the incorporation of P₂O₅ in the glass network is highly effective at reducing Young's modulus and shear modulus. Incorporating P₂O₅ in the glass network also reduces the high temperature CTE, increases the ion-exchange interdiffusion rate, and improves glass compatibility with zircon refractory materials. In some embodiments, the glasses described herein comprise from about 1 mol % to about 8 mol % P₂O₅.

Boron oxide (B₂O₃) is also a glass-forming oxide that is used to reduce viscosity and thus improves the ability to melt and form glass. B₂O₃ can exist in either threefold or fourfold coordination in the glass network. Threefold coordinated B₂O₃ is the most effective oxide for reducing the Young's modulus and shear modulus, thus improving the intrinsic damage resistance of the glass. Accordingly, the glasses described herein, in some embodiments, comprises from about 2 mol % to about 10 mol % B₂O₃ and, in other embodiments, from about 5 mol % to about 8 mol % B₂O₃. The presence of both B₂O₃ and P₂O₃ in the glass enhances the mechanical performance of the glass by increasing the inherent damage resistance (IDR) of the glass. When ion exchanged, the glasses described herein exhibit Vickers indentation thresholds ranging from about 10 kgf to about 20 kgf.

The alkali oxide Na₂O is used to achieve chemical strengthening of the glass by ion exchange. The glasses described herein include Na₂O, which can be exchanged for potassium in a salt bath containing, for example, KNO₃. In some embodiments, the glass comprises from about 14 mol % to about 20 mol % Na₂O and, in other embodiments, from about 15 mol % to about 20 mol % Na₂O.

Like B₂O₃, the divalent oxide ZnO also improves the melting behavior of the glass by reducing the temperature at 200 poise viscosity (200P temperature). ZnO also is beneficial in improving the strain point when compared to like additions of P₂O₅, B₂O₃, and/or Na₂O. In some embodiments, the glasses described herein comprise from about 1 mol % up to about 7 mol % ZnO and, in other embodiments, from about 2 mol % to about 5 mol % ZnO.

Alkaline earth oxides, including MgO and CaO, may also be substituted for ZnO to achieve a similar effect on the 200P temperature and strain point. When compared to MgO and CaO, however, ZnO is less prone to promoting phase separation in the presence of P₂O₅. Other alkaline earth oxides, including SrO and BaO, may also be substituted for ZnO, but are less effective in reducing the melt temperature at 200 poise viscosity than ZnO, MgO, or CaO and are also less effective than ZnO, MgO, or CaO at increasing the strain point.

In some embodiments, the base glasses described herein are formable by down-draw processes that are known in the art, such as slot-draw and fusion-draw processes. The base glass compositions containing small concentrations of Li₂O are fully compatible with the fusion-draw process and can be manufactured without issue. The lithium can be batched as either spodumene or lithium carbonate.

The fusion draw process is an industrial technique that has been used for the large-scale manufacture of thin glass sheets. Compared to other flat glass manufacturing techniques, such as the float or slot draw processes, the fusion draw process yields thin glass sheets with superior flatness and surface quality. As a result, the fusion draw process has become the dominant manufacturing technique in the fabrication of thin glass substrates for liquid crystal displays, as well as for cover glass for personal electronic devices such as notebooks, entertainment devices, tables, laptops, and the like.

The fusion draw process involves the flow of molten glass over a trough known as an “isopipe,” which is typically made of zircon or another refractory material. The molten glass overflows the top of the isopipe from both sides, meeting at the bottom of the isopipe to form a single sheet where only the interior of the final sheet has made direct contact with the isopipe. Since neither exposed surface of the final glass sheet has made contact with the isopipe material during the draw process, both outer surfaces of the glass are of pristine quality and do not require subsequent finishing.

In order to be fusion drawable, a glass must have a sufficiently high liquidus viscosity (i.e., the viscosity of a molten glass at the liquidus temperature). In some embodiments, the glasses described herein have a liquidus viscosity of at least about 100 kilopoise (kpoise), in other embodiments, at least about 120 kpoise, and in still other embodiments, these glasses have a liquidus viscosity of at least about 300 kpoise.

While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the disclosure or appended claims. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present disclosure or appended claims. 

1. A glass article, the glass article comprising at least about 50 mol % SiO₂; at least about 8 mol % Al₂O₃; at least about 1 mol % P₂O₅; and at least about 12 mol % Na₂O, wherein the glass article is ion exchangeable and has a softening point of less than or equal to about 825° C., and a high temperature coefficient of thermal expansion of less than or equal to 30 ppm/° C.
 2. The glass article of claim 1, wherein the glass article is ion exchanged and has a compressive layer extending from a surface of the glass article to a depth of layer within the article, the compressive layer having a maximum compressive stress of at least about 700 MPa.
 3. The glass article of claim 2, wherein the maximum compressive stress is at least about 800 MPa.
 4. The glass article of claim 3, wherein the maximum compressive stress is at least about 900 MPa.
 5. The glass article of claim 2, wherein the depth of layer is at least about 20 μm.
 6. The glass article of claim 2, wherein the glass has a Vickers crack initiation threshold of at least about 10 kgf.
 7. The glass article of claim 1, wherein the glass article comprises: from about 50 mol % to about 65 mol % SiO₂; from about 10 mol % to about 20 mol % Al₂O₃; from about 1 mol % to about 8 mol % P₂O₃; from about 2 mol % to about 10 mol % B₂O₃; and from about 14 mol % to about 20 mol % Na₂O.
 8. The glass article of claim 7, wherein the glass article further comprises from about 1 mol % to about 7 mol % ZnO.
 9. The glass article of claim 7, wherein Al₂O₃(mol %)>P₂O₅(mol %)+B₂O₃(mol %), Al₂O₃(mol %)>B₂O₃(mol %), and Na₂O(mol %)>Al₂O₃(mol %).
 10. The glass article of claim 7, wherein 0.3≦[(R₂O(mol %)+RO(mol %)+B₂O₃(mol %)]/[(Al₂O₃(mol %)+P₂O₅ (mol %)+SiO₂ (mol %)]≦0.45, where R₂O are monovalent cation oxides and RO are divalent cation oxides.
 11. The glass article of claim 1, wherein the glass article is substantially free of at least one of lithium, potassium, and alkaline earth metals.
 12. The glass article of claim 1, wherein the softening point is less than about 800° C.
 13. The glass article of claim 1, wherein the softening point is less than about 775° C.
 14. The glass article of claim 1, wherein the glass article has a non-planar shape.
 15. The glass article of claim 1, wherein the glass article is down drawable.
 16. An glass article, the glass article comprising at least about 50 mol % SiO₂, at least about 8 mol % Al₂O₃, at least about 1 mol % P₂O₅, and at least about 12 mol % Na₂O, wherein the glass article is ion exchanged and has a compressive layer extending from a surface of the glass article to a depth of layer within the article, the compressive layer having a maximum compressive stress of at least about 700 MPa, and wherein the glass has a softening point of less than or equal to about 825° C. and a high temperature coefficient of thermal expansion of less than or equal to 29 ppm/° C.
 17. The glass article of claim 16, wherein the maximum compressive stress is at least about 800 MPa.
 18. The glass article of claim 17, wherein the maximum compressive stress is at least about 900 MPa.
 19. The glass article of claim 16, wherein the depth of layer is at least about 20 μm.
 20. The glass article of claim 16, wherein the glass has a Vickers crack initiation threshold of at least about 10 kgf.
 21. The glass article of claim 16, wherein the glass article comprises: from about 50 mol % to about 65 mol % SiO₂; from about 10 mol % to about 20 mol % Al₂O₃; from about 1 mol % to about 8 mol % P₂O₃; from about 2 mol % to about 10 mol % B₂O₃; and from about 14 mol % to about 20 mol % Na₂O.
 22. The glass article of claim 21, wherein the glass article further comprises from about 1 mol % to about 7 mol % ZnO.
 23. The glass article of claim 21, wherein Al₂O₃(mol %)>P₂O₅(mol %)+B₂O₃(mol %), Al₂O₃(mol %)>B₂O₃(mol %), and Na₂O(mol %)>Al₂O₃(mol %).
 24. The glass article of claim 21, wherein 0.3≦[(R₂O(mol %)+RO(mol %)+B₂O₃(mol %)]/[(Al₂O₃(mol %)+P₂O₅ (mol %)+SiO₂ (mol %)]≦0.45, where R₂O are monovalent cation oxides and RO are divalent cation oxides.
 25. The glass article of claim 16, wherein the glass article is substantially free of at least one of lithium, potassium, and alkaline earth metals.
 26. The glass article of claim 16, wherein the softening point is less than about 800° C.
 27. The glass article of claim 16, wherein the softening point is less than about 775° C.
 28. The glass article of claim 16, wherein the glass article has a non-planar shape.
 29. The glass article of claim 16, wherein the glass article is down drawable. 